On the front of the tank, where the fuel line connects to, you need to take that fitting out and there will be a hose attached to it with the filter at the end. Make sure you put a filter designed for that back in it's place, it has a check valve so the gas doesn't drain out of the line.
Or, if your a good fisherman you can fish around with a clothes hanger or something and try and get it through the filler cap hole.